I didn't say it was easy, I was actually meaning the time between the displaying of the weapon and the first shot. Some of the nuts have something to say leading up to the shooting or brandish their gun for a short period prior to the actual event-some don't. The threatening/brandishing period is when they're building up and could possibly be taken down. The pull it and shoot it ones would be hard to predict but the one thing they have in their mind is that everyone will run, hide, or fold. Something happening counter to what they expect might throw them off their premeditated plan.
While it would be unsettling to have a miss or pass through hitting an innocent, stopping the possibilty of a larger number of people dead or injured would seem to be preferable(minus the liability lawyers lining up like buzzards on a possum carcass).